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Department of Computer Science Southern Illinois University Carbondale

Department of Computer Science Southern Illinois University Carbondale CS591 – Wireless & Network Security Lecture 1: Communication Basics. Dr. Kemal Akkaya E-mail: kemal @cs.siu.edu. Transmission Fundamentals. 1 Cycle. Wavelength. Amplitude. Analog: No breaks in the signal.

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Department of Computer Science Southern Illinois University Carbondale

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  1. Department of Computer Science Southern Illinois University Carbondale CS591 – Wireless & Network Security Lecture 1: Communication Basics Dr. Kemal Akkaya E-mail: kemal@cs.siu.edu Wireless & Network Security 1

  2. Transmission Fundamentals 1 Cycle Wavelength Amplitude Analog: No breaks in the signal Digital: Signal intensity is discrete • How to relay information? • Electromagnetic Signals • TV, Radio, Internet etc. • Signal • A function of time • Has 3 components: • Amplitude (A) : Signal strength • Frequency (f) : # of cycles • Phase ( ) : Relative position • Sine wave for the signal • s(t) = A sin(2 f t +  ) • Either analog or digital • Wavelength (λ): • Distance occupied by 1 cycle • λ = c*T = c / f Wireless & Network Security 2

  3. Effects on Signal • Attenuation: • Decrease in amplitude of signal along transmission • Distortion: • Interference of different frequency components of a signal • Noise: • In the absence of signal, there is random mixture of frequencies on the channel called channel noise • Error: • When digital signals are combined with noise, some bits can be received in error Wireless & Network Security 3

  4. Signal/Bandwidth/Data Rate sin(2ft)+(1/3) sin(23ft) • Signal may include many frequencies • Combination of sinusoids • Spectrum: • Range of frequencies a signal contains • The signal in the figure contains frequencies between f and 3f • Bandwidth: • Width of the spectrum is called bandwidth • Bandwidth for the figure : 3f – f = 2f • Increasing the bandwidth makes the wave look like more square (i.e. digital signal) • Hence, increasing the bandwidth helps to reduce the distortion at the receiver side. Bandwidth = 7f – f = 6f How much data can we communicate with a certain bandwidth? Wireless & Network Security 4

  5. Analog and Digital Data Transmission What does a Modem do? ? • How analog and digital signals are transmitted? • Analog signals (continuous) can be propagated through • Wire, twisted pair, coaxial cable, fiber optic cable and atmosphere • Digital signals (discrete) can only propagated through • Wired medium – No wireless since it requires infinite frequencies • How to propagate digital signals then? • Digital data can be represented as analog signals: Wireless & Network Security 5

  6. How to do that encoding? • Modulation is the solution: • Modulate digital data so that an analog signal is generated • Modem would be the classical example • Motivation: When only analog transmission facilities are available, modulation is required to convert digital data into analog signals • How to do digital modulation? • Operation in on or more of the 3 characteristics of a signal • These are amplitude, frequency and phase • Three main techniques • ASK: Amplitude Shift Keying – digital data over optical fiber • FSK: Frequency Shift Keying – on LANs that use coaxial cable • PSK: Phase Shift Keying – 802.11 Networks Wireless & Network Security 6

  7. ASK, FSK and PSK Wireless & Network Security 7

  8. Other digital modulation techniques • Binary Frequency Shift Keying (BFSK) • Uses two different frequencies • Multiple Frequency Shift Keying (MFSK) • More than two frequencies are used • Gaussian Frequency Shift Keying (GFSK) • Two level shift from base frequency : Bluetooth uses this • Binary Phase Shift Keying (BPSK) • Two phrases used to represent bits : In Satellite Systems • Differential Phase Shift Keying (DPSK) • Phase shift with reference to previous bit • Four-level (QPSK) and Multilevel Phase Shift Keying • Each element represents more than 1 bit • Differential QPSK (DQPSK) is used in 802.11b networks • Quadrature Amplitude Modulation (QAM) • Combination of ASK and PSK • Two different signals sent simultaneously on the same carrier frequency • Started to be used in Wireless Sensor Networks Wireless & Network Security 8

  9. AM and FM Example FM AM Wireless & Network Security 9

  10. Digitization 7D/2 5D/2 3D/2 D/2 -D/2 -3D/2 -5D/2 -7D/2 • Converting analog data into digital signals • Digital data can then be transmitted using NRZ-L • NRZ-L a way to transmit digital signals • Digital data can then be transmitted using code other than NRZ-L • Digital data can then be converted to analog signal • Analog to digital conversion done using a codec • Pulse Code Modulation (PCM) • Delta Modulation (DM) Original signal Sample value Approximation 3 bits / sample Rs = Bit rate = # bits/sample x # samples/second Wireless & Network Security 10

  11. Multiplexing • Carrying multiple signals on a single medium • Capacity of transmission medium usually exceeds capacity required for transmission of a single signal • More efficient use of transmission medium: • Combine multiple signals • Increased data rate provides cost efficiency • Transmission and reception equipment • Analog multiplexing • Frequency Division Multiplexing (FDM) • Digital Multiplexing • Time Division Multiplexing (TDM) Wireless & Network Security 11

  12. FDM Example 3 Channels 1 Link Transmission Multiplexer • Combining analog signals • Takes advantage of the fact that the useful bandwidth of the medium exceeds the required bandwidth of a given signal Wireless & Network Security 12

  13. TDM Example • Digital technique to combine data • Takes advantage of the fact that the achievable bit rate of the medium exceeds the required data rate of a digital signal Wireless & Network Security 13

  14. Transmission Media for Signals • It is the physical path between transmitter and receiver • Guided media: Solid media such as copper, optical fiber etc. • Unguided media: Atmosphere or outer space: Wireless Transmission • Here is the electromagnetic spectrum for telecommunications: Wireless & Network Security 14

  15. General Frequency Ranges Electric Waves Radio Waves Visible Light Ultra Violet Gamma Rays Cosmic Rays Infra-red X-Rays Radio Spectrum “Sweetspot” 3G LMDS WiFiBluetooth DECT TETRA GSM FM Radio Medium Wave Radio Microwave Radio Links Long Wave Radio TV VLF LF MF HF VHF UHF SHF EHF 3 30 300 3 30 300 3 30 300 kHz MHz GHz • Infrared frequency range • Roughly 3x1011 to 2x1014 Hz • Useful in local point-to-point multipoint applications within confined areas • Microwave frequency range • 1 GHz to 40 GHz • Used for satellite communications • Radio frequency range • 3 KHz to 300 GHz • Can be analog : TV, Radio • Or digital: Cell phones, wireless networks Wireless & Network Security 15

  16. Frequency Regulations • Federal Communications Commission (FCC) • Charged with regulating interstate and international communications by radio, television, wire, satellite and cable • Prevent interferences between different devices Current Allocation of the Radio Spectrum by frequency Wireless & Network Security 16

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